Practical application of a fuel cell that supplies hydrogen as a fuel gas to a fuel electrode, and that supplies air as an oxidant gas to an oxidant electrode, and that generates electricity through an electrochemical reaction between hydrogen and oxygen in the air while producing water on an oxidant electrode is now being considered.
In such a fuel cell, if at the time of start of operation, the pressure of hydrogen supplied to the fuel electrode and the pressure of air supplied to the oxidant electrode are about equal to the respective pressures occurring during ordinary operation, it sometimes happens that hydrogen gas and air are unevenly distributed in the fuel electrode and the oxidant electrode, respectively, and the electrodes are degraded by electrochemical reaction caused by the uneven distribution of these gases. Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method of preventing the degradation of the electrodes of a fuel cell by causing the pressures of hydrogen and air supplied to the fuel electrode and the oxidant electrode, respectively, at the time of start of operation of the fuel cell to be higher than the ordinary supplied pressures of these gases.
However, if hydrogen gas and air are supplied at high pressure to a fuel cell when the fuel cell starts operation, it sometimes happen that the rate of rise of the voltage of the fuel cell becomes large so that the voltage of the fuel cell overshoots its upper-limit voltage. In conjunction with this problem, Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method in which when hydrogen gas and air are supplied, at the time of starting a fuel cell, at pressures that are higher than their pressures given during ordinary power generation, output electric power is extracted from the fuel cell, and is put out to a vehicle driving motor, resistors, etc., provided that the voltage of the fuel cell reaches a predetermined voltage that is lower than the upper-limit voltage.
By the way, since the fuel cell uses hydrogen as a fuel gas, it is necessary to check that there is no leakage of hydrogen when the fuel cell is started. To this end, a method in which the presence/absence of hydrogen leakage from the system is determined by sealing the hydrogen system and then checking whether or not the pressure in the system becomes low. However, during the state where hydrogen and oxygen in air are undergoing an electrochemical reaction within the fuel cell, the hydrogen supplied to the fuel cell is consumed by the electrochemical reaction. Therefore, the pressure of the hydrogen system that is sealed decreases even when there is no leakage of hydrogen, and the hydrogen leakage sometimes cannot be accurately determined. Therefore, in a related-art technology as shown in FIG. 8, after an ignition key is turned on at time t0′, the control value of the output voltage of the fuel cell shown by a line a′ is set at an open-circuit voltage OCV. At time t1′, the voltage of the fuel cell starts to be raised as shown by a line b′ by supplying hydrogen and oxygen are supplied to the fuel cell, and therefore pressurizing a hydrogen system and an oxygen system. As a result, the voltage of the fuel cell is temporarily raised to the open-circuit voltage OCV. Then, in the related-art method, it is detected whether or not there is hydrogen leakage, during a period from time t2′ to time t3′ during which the voltage of the fuel cell remains at the open-circuit voltage OCV. When the voltage of the fuel cell reaches the open-circuit voltage OCV, the electrochemical reaction between hydrogen and oxygen within the fuel cell does not progress any longer, so that the hydrogen in the sealed hydrogen system is not consumed. Therefore, a state in which the pressure of the sealed hydrogen system undergoes hardly any decrease can be created if there is no leakage of hydrogen. Then, it can be determined whether or not there is hydrogen leakage by detecting the degree of pressure decrease of the hydrogen system during the foregoing state. However, when the voltage of the fuel cell reaches the open-circuit voltage OCV, the durability of the fuel cell can be adversely affected.